A fire broke out in Washington's subway system Saturday night, producing heavy smoke but no injuries.

Authorities said an insulator caught fire in a mechanical room, which produced smoke in the Friendship Heights station and the Tenleytown/American University station. Rescue crews evacuated the stations but reported no injuries.

Washington's subway system, known as the Metro, has been under scrutiny in recent years following a series of fires and safety incidents.

A fire in 2015 killed one subway rider and injured several others. Last month, another fire led officials to shut the system down for an entire day for emergency maintenance. That was the first such emergency maintenance shutdown since the Metro opened in 1976.

Metro officials have suggested that they may need to shut down some rail lines for weeks or months at a time in order to complete important repairs.